Electrical insulation applications are generally divided into low voltage insulation which are those less than 5K volts, medium voltage insulation which ranges from 5K volts to 60K volts, and high voltage insulation, which is for applications above 60K volts.
For medium voltage cable applications, the most common polymeric insulators are made from an ethylene-based polymer, typically either from polyethylene or ethylene-propylene elastomers, otherwise known as ethylene-propylene-rubber (EPR). The polyethylene can be any one or more of a number of various polyethylenes, e.g., homo- or copolymer, high density polyethylene (HDPE), high pressure low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and the like. The polyethylenes are typically crosslinked, usually through the action of a peroxide, but are still prone to treeing, particularly water treeing.
Water treeing is the deterioration of a solid dielectric material which is simultaneously exposed to moisture and an electric field. It is a significant factor in determining the useful life of buried power cables. Water trees initiate from sites of high electrical stress such as rough interfaces, protruding conductive points, voids, or imbedded contaminants but at a lower field than that required for electrical trees. In contrast to electrical trees, water trees are characterized by: (a) the presence of water (which is essential for their growth); (b) long term growth (they can grow for years before reaching a size at which they may contribute to a breakdown); and (c) growth is initiated and continued in a much lower electrical field than that required for the initiation and growth of electrical trees.
Water treeing is accepted as a factor in wet-electrical aging of insulation in power cable designs which are not water impervious. Water tree retardancy of insulation materials is brought about by the use of additives such as polyethylene glycol (PEG) or the incorporation of a polar ethylene copolymer, e.g., ethylene vinyl acetate (EVA), ethylene ethyl acrylate (EEA) and the like.
Thermo-oxidative damage during long term aging is another mode of failure of medium voltage cables. Retardancy of this unwanted phenomenon is typically achieved through a combination of peroxide-initiated crosslinking the ethylene-based polymer and the use of a sulphur containing hindered phenol stabilizer. Hindered amine stabilizers are known for their use as thermo-oxidative stabilizers and inhibitors of electrical trees, a form of electrical degradation. However, a known antagonism exists between amine and sulphur based molecules that restrict their combined use in systems that required both good long term heat aging retardancy and water-tree and electrical-tree resistance. Moreover, PEG is a known pro-degradant with respect to thermo-oxidative stability in tree resistant, crosslinked polyethylene (TRXLPE) insulation, and thus these systems require a greater degree of stabilization than is required of non-tree resistant crosslinked polyethylene insulation.